Many large and small commercial industries produce waste gases or exhaust air that contain environmentally objectionable contaminants. Fumes such as solvents and other hydrocarbon substances, generally referred to as VOC, include gasoline vapors, paint fumes, chlorinated hydrocarbons. The most common method of eliminating such combustible fumes prior to emitting the exhaust gases to the atmosphere is incineration.
One method of incineration passes the waste gas or exhaust air stream through a fume incinerator prior to venting. U.S. Pat. No. 4,444,735 discloses a fume incinerator for incinerating combustible fumes in an oxygen bearing process exhaust stream. The process gas stream is passed through a flame front in the incinerator produced from burning fossil fuel, typically natural gas or fuel oil. In order to insure complete incineration of the combustible contaminants, the entire process exhaust stream must pass through the flame front. It is often necessary to preheat the process exhaust stream prior to contacting it with the flame front. The cost of the preheat heat exchanger and the auxiliary fuel make fume incinerators relatively expensive.
Multiple-bed, fossil fuel-fired regenerative incinerator, such as incinerators disclosed in U.S. Pat. Nos. 3,870,474 and 4,741,690 are also commonly used. Multiple-bed systems usually employ two or more regenerative beds of heat-accumulating and heat-transferring material disposed about a central combustion chamber equipped with a fossil fuel-fired burner. The process exhaust stream to be incinerated is passed through a first bed, then into a central combustion chamber for incineration in the flame produced by supplemental fuel and then discharged through a second bed. As the hotter incinerated process exhaust stream passes through the second bed, it loses heat to the bed material. After a period of time, the direction of gas flow through the system is reversed and the incoming process exhaust stream then passes first through the second bed, thereby preheating the incoming stream, then through the central combustion chamber, and then through the first bed. By periodically reversing the direction of gas flow, the incoming process exhaust stream is preheated by heat stored from the previously incineration cycle, thereby regenerating heat and reducing supplemental fuel requirements.
Usually regenerative thermal oxidizers control combustion zone temperature, or VOC destruction temperature, by adding supplementary fuel usually through a burner in the combustion zone or by adding fuel directly to the VOC process exhaust stream, when the VOC load decreases. If the VOC load increases and the combustion zone temperature rises above set-point, the supplementary fuel is closed off. If the combustion zone temperature continues to increase, makeup or purge air is added to cool the incinerator enough to prevent damage thereto. The purge air reduces process efficiency by increasing fan power cost. Switching flow directions entering and exiting the beds is usually performed on a fixed, timed sequence or schedule. In the past in such systems the temperature of the gas vented through the switching valve to the atmosphere after incineration has been maintained at a constant temperature, e.g. about 225.degree. F. When the VOC loading drives the combustion temperature up, purge air has been added to lower the combustion temperature by allowing a larger quantity of hot vented gas to leave the oxidizer at about 225.degree. F.
Another method of controlling vent gas temperature is disclosed in U.S. Pat. No. 5,186,901 in which exhaust gases are recirculated for cooling purposes. Recirculating exhaust gas or adding purge air reduces process efficiency by increasing fan power.
Examples of switching valves for regenerative incinerator systems are disclosed in U.S. Pat. Nos. 3,770,050, and 4,909,307.